Registers an Authenticator with the ESMTPClient. The ESMTPClient will
attempt to login to the SMTP Server in the order the Authenticators are
registered. The most secure Authentication mechanism should be registered
first.

This may be considered the initializer of the protocol, because it is
called when the connection is completed. For clients, this is called once
the connection to the server has been established; for servers, this is
called after an accept() call stops blocking and a socket has been
received. If you need to send any greeting or initial message, do it
here.

The step may be to ask the server to being a TLS session. If TLS is
already in use or not necessary and not available then the step may be to
authenticate with the server. If TLS is necessary and not available, fail
the mail transmission attempt.

This is an internal helper method.

Parameters

code

The server status code from the most recently received server message. (type: int)

resp

The server status response from the most recently received server message. (type: bytes)

items

A mapping of ESMTP extensions offered by the server. Keys are extension
identifiers and values are the associated values. (type: dict
mapping bytes
to bytes)